From the Pile Driving Contractors Association’s “Installation Specification for Driven Piles”:
Pile driving hammers shall be of the size needed to develop the energy required to drive the piles at a blow count that does not exceed 10 blows per inch at the required ultimate pile capacity. The intent is to select the size of hammer at normal operating condition to be sufficient. Drillers sometimes may be required to drive to a higher blow count to penetrate an unforeseen thin, dense layer or minor obstruction. Jetting or drilling may be preferred means to penetrate a particularly dense layer. Overdriving often will damage the pile and/or hammer.
Drop HammersDrop hammers shall not be used for concrete piles or for piles whose required ultimate pile capacity exceeds 60 tons. When drop hammers are permitted, the ram shall have a weight not less than 1 ton, and the height of drop shall not exceed 12 feet. In no case shall the ram weight of drop hammers be less than the combined weight of helmet and pile. Lighter drop weights might be insufficient to spool the crane winch. All drop hammers shall be equipped with hammer guides and a helmet to ensure concentric impact.
Air HammersIf a dynamic formula is used to establish the required blow count, the weight of the striking parts of air hammers used shall not be less than one-third the weight of pile and drive cap, and in no case shall the striking part have a weight less than 1.4 tons. If a wave equation analysis is used to establish the required blow count and driving stresses, this limitation on ram weight does not apply. Smaller ram-weight hammers can be used for special applications.
The plant and equipment furnished for air hammers shall have sufficient capacity to maintain – under working conditions – the pressure at the hammer specified by the manufacturer. The hose connecting the compressor with the hammer shall be at least the minimum size recommended by the manufacturer.
Hammer performance shall be evaluated at the end of driving by measuring blows per minute, and then comparing with the manufacturer’s recommendations.
Diesel HammersSome open-end (single-acting) diesel hammers are equipped with a device to measure impact velocity at all times during pile driving operations. If this device is not available, the stroke shall be obtained by measuring the speed of operation – either manually or with a device that makes the measurement automatically. It is important to record stroke or bounce-chamber pressure with the blow count.
Closed-end (double-acting) diesel hammers shall be equipped with a bounce-chamber pressure gauge in good working order, mounted near ground level so as to be easily read. The drilling contractor shall provide a correlation chart of bounce-chamber pressure and potential energy.
Either impact velocity or stroke measurement is required and should be recorded. Jump sticks to visually measure stroke should not be used for safety reasons.
Hydraulic HammersHydraulic hammers shall be equipped with a system for measurement of ram energy. The system shall be in good working order, and the results shall be easily and immediately available.
The measurement of impact velocity makes it possible to calculate the kinetic energy of the ram at impact. The measurement device may display either impact velocity or energy. Record this information with the blow count.
Vibratory HammersVibratory or other pile driving methods may be used only when specified in the contract documents or in writing by the engineer. Except when pile lengths have been evaluated from load test piles, the ultimate pile capacity of piles driven with vibratory hammers shall be verified by redriving the first pile driven in each group of 10 piles with an impact hammer of suitable energy to measure the ultimate pile capacity before driving the remaining piles in the group. All piles that rely on point-bearing capacity shall be redriven with an impact hammer.
In case the required penetration is not obtained by the use of a hammer complying with the minimum requirements above, the drilling contractor may be required to provide a hammer of greater energy or, when permitted, resort to supplemental methods such as jetting or predrilling.